Various methods are known of coating non-conductive surfaces. In wet chemical methods, the surfaces to be metallised are, after an appropriate preliminary treatment, either firstly catalysed and then metallised in an electroless manner and thereafter, if necessary, metallised electrolytically, or are directly electrolytically metallised.
Methods according to the first variant with electroless metallisation have, however, proved disadvantageous, as process management of the electroless metallising bath is difficult, treatment of the waste water from this bath is complex and expensive, and the process is lengthy and thus likewise expensive due to the low deposition speed of the metallising bath.
Especially for metal coating of plastic parts, for example for sanitary fittings and for the automobile industry, and of parts which are used as casings for electrical appliances which are screened against electromagnetic radiation, the electroless metallising methods are problematic. In treatment of such moulded parts, generally relatively large volumes of the treatment solutions are carried over from one treatment bath into the next, as these have a shape by means of which the treatment solution is transported out of the baths when the parts are lifted out. As electroless metallising baths normally contain considerable quantities of toxic formaldehyde and complex formers which are only removable with difficulty, in their treatment large quantities of these baths are lost and must be disposed of in a complicated manner.
For this reason a series of metallising methods was developed, by means of which the non-conductive surfaces could be directly coated with metal without electroless metallisation (see, for example, EP 0 298 298 A2, U.S. Pat. No. 4,919,768, EP 0 320 601 A2, U.S. Pat. No. 3,984,290, EP 0 456 982 A1 and WO 89/08375 A1).
In EP 0 616 053 A1 there is disclosed a method for direct metallisation of non-conductive surfaces, in which the surfaces are firstly treated with a cleaner/conditioner solution, thereafter with an activator solution, for example a palladium colloidal solution, stabilised with tin compounds, and are then treated with a solution which contains compounds of a metal which is more noble than tin, as well as an alkali hydroxide and a complex former. Thereafter the surfaces can be treated in a solution containing a reducing agent, and can finally be electrolytically metallised.
WO 96/29452 concerns a process for the selective or partial electrolytic metallisation of surfaces of substrates made from electrically non-conducting materials which for the purpose of the coating process are secured to plastic-coated holding elements. The proposed process involves the following steps: a) preliminary treatment of the surfaces with an etching solution containing chromium (VI) oxide; followed immediately by b) treatment of the surfaces with a colloidal acidic solution of palladium-/tin compounds, care being taken to prevent prior contact with adsorption-promoting solutions; c) treatment of the surfaces with a solution containing a soluble metal compound capable of being reduced by tin (II) compounds, an alkali or alkaline earth metal hydroxide, and a complex forming agent for the metal in a quantity sufficient at least to prevent precipitation of metal hydroxides; d) treatment of the surfaces with an electrolytic metallisation solution.
The processes described in EP 0 616 053 A1 and WO 96/29452 are disadvantageous in that they require the use of a noble metal such as palladium which is a very expensive metal.
Hence, it is the object underlying the present invention to provide a process requiring a reduced amount of a noble metal such as palladium to activate the surface of the non-conductive substrate to be metal-coated.